1. Field of the Invention
This invention relates generally to a communications system and, more particularly, to an improved antenna nulling system for suppressing jammer signals.
2. Discussion of the Related Art
Various types of adaptive antenna control systems have been developed to counteract jamming signals. In general, adaptive antenna control systems respond to a jamming signal by forming a null in the antenna pattern, greatly reducing the gain in the direction in which the jamming signal appears.
A number of mathematical techniques known as nulling algorithms have been used to analyze jamming signals and produce effective nulls. These nulling algorithms each have advantages and disadvantages depending on the nulling criteria. For example, most nulling algorithms perform both a jammer suppression function to null the jamming signals and a pattern constraint function to maintain the antenna pattern or coverage area for continued communication in the antenna coverage area.
One such system is disclosed in U.S. Pat. No. 5,175,558 to DuPree, which is hereby incorporated by reference. In DuPree, the nulling algorithm performs jammer suppression using a gradient approach and pattern constraint using a difference in weights approach. The gradient approach utilizes a relatively simple hardware configuration which reduces the overall hardware costs with the tradeoff of having a slower convergence rate for nulling the jamming signals. The difference in weights pattern constraint is also relatively simple and not very computationally tasking, but has a trade off of generally nulling a larger coverage area. This reduced coverage area may eliminate reception of communications from users operating adjacent to where the nulling signals are formed.
Another antenna nulling algorithm is provided by MIT Lincoln Labs which is known as the PACE algorithm (Post Adaptive Coverage Enhancement) or the S-PACE algorithm (Superior Post Adaptive Coverage Enhancement). Each of these antenna nulling algorithms again provide for both jammer suppression and pattern constraint. The jammer suppression is provided using a null space approach, also known as Eigen-vector nulling. This jammer suppression approach provides an increased convergence rate at the cost of increased hardware complexity. The pattern constraint for these nulling algorithms is performed either using a difference in pattern approach (PACE) or a difference in pattern magnitude approach (S-PACE). The difference in pattern or the difference in pattern magnitude approach attempts to maintain the antenna pattern as close to the quiescent antenna pattern or original antenna coverage area, thereby providing sharper nulling in the antenna pattern or coverage area. The trade off in these pattern constraint techniques is that they are more computationally tasking then the difference in weights approach. Therefore, the PACE and S-PACE systems employ both complex jammer suppression and pattern constraint.
What is needed then is an improved antenna nulling system for nulling jamming signals which does not suffer from the above-mentioned disadvantages. This will, in turn, provide an improved antenna nulling system that produces sharp nulls at the antenna jamming signals but does not degrade desired incoming communications signals from multiple users. Such a system will balance the disadvantages of the various nulling algorithms to provide a nulling algorithm that provides superior results with reduced hardware complexity and costs to provide overall improved performance. It is, therefore, an object of the present invention to provide an improved antenna nulling system which utilizes a simpler gradient approach to suppress the jamming signals and a difference in pattern magnitude approach to recover lost antenna signal gain due to suppression of the received jamming signals. The gradient approach will iteratively readjust the magnitude and the angle of received signals from each antenna feed to suppress the overall jammer signal strength. The difference in pattern magnitude approach will increase gain around the removed jamming signal to increase signal strength to users in the area.